It has been said that some of his initial observations were made using a drop of water as a magnifying lens…

This sounds like a far-fetched story until one thinks about the ease with which a small drop of water can be produced, compared to the difficulty of making a glass lens of the same size. The magnifying power of a lens is inversely proportional to the radius of curvature of its surface. Therefore, the smaller the radius of the water drop, the higher its optical magnification will be. Careful observation after a rainy day will persuade us that the water drop can actually be used as a powerful single-lens microscope.

There is of course, the practical challenge of holding that small drop of water very close in front of your eye, and putting the sample to be observed on the other side. Luckily for us today, by mixing very old and very new technology, we can get the best of both worlds and verify some of Leeuwenhoek’s observations.

Here is what you need:

A cup of water

A cotton swab

An iPad (or any other tablet-like device with a front-facing camera)

The iPad 2 has a front camera that is protected by the same cover glass that extends through the rest of the front surface. This is great for us, because we can simply:

Place the iPad on a flat surface

Immerse the cotton swab in the cup of water

Squeeze the cotton swab head on top of the iPad camera until a drop of water falls on top of the cameraThis may require several tries, since you want to get:

The smallest possible drop

The drop centered on top of the camera lens

The result would look similar to the images below:

Once you have placed the drop, turn on the camera app and select to use the front camera.

By placing a thick book beside the iPad, we get a stable platform in which we can insert samples to hold them in front of the camera. By moving the sample to different pages in the book, we get a high-precision sample holder. The book that we used here is 35mm thick and has 600 pages, which gives us a resolution of 0.05mm (50 microns) on the height at which to place the sample above the water drop lens. This precision turns out to be very important because the focal length of the water drop is about 1.5X the radius of the drop.

Here we place a microscopy slide containing a cross-section of a Tilia stem.

and here is the corresponding image taken with the iPad camera.

It is a bit challenging that the button that must be pressed to take the picture in the iPad is just beside the camera, and therefore ends up just underneath the microscopy slide. To avoid this, you may want to tilt the iPad to get it landscape orientation first, and then place it in the table to use it for observation.

The optics of the water drop are quite good for macro observations. Time to hunt for household objects to take a closer look at them…

Here is a pencil:

and the resulting image taken with the iPad camera:

The water drop used here was rather large (about 5mm in diameter), and therefore produces a magnification of just about 20X.

This is good enough for certain interesting observations, for example… when was the last time that you cleaned up your computer mouse ?

Micrographia, by Robert Hooke: Download available at the Gutenberg project: http://www.gutenberg.org/ebooks/15491 (Now in the Public Domain). Note that at the time of the publication of this book (1665), copyright in England was regulated by the Licensing Act of 1662 in the form of giving an exclusive monopoly on printing, to the Stationers Company. Authors did not have a copyright per se, based on a recognition to their creative works. Instead the Stationers Company enjoyed this monopoly in exchange for assisting on the censorship of texts that might be offensive or critical to the crown. The Commons refused to renew the printer’s licensing act in 1695, and in 1710, under Queen Anne, England introduced the first Copyright Act, “…for the encouragement of learning…”, that gave authors the right control the reproduction of their writings, for a limited time. In the case of Micrographia, Hooke got a copyright for 21 years, and therefore the book went into the public domain in 1686.

Leeuwenhoek letters to the Royal Society:“Observations, communicated to the Publisher by Mr Antony van Leewenhoeck, in a Dutch letter of the 9th of October 1676. Here Englishe’d: Concerning little animals by him observed in Rain-Well-Sea. and Snow water; and also Water Wherein Pepper had lain infused.”http://rstl.royalsocietypublishing.org/cgi/doi/10.1098/rstl.1677.0003Available in Open Access.